The present invention relates to a receiving circuit or a receiver for receiving an RF signal and more particularly to a manner in which an oscillator for oscillating a reference signal used to determine a receiving frequency is controlled.
FIG. 1 of the accompanying drawings shows an example of a receiving circuit.
As shown in FIG. 1, an RF signal received at an antenna 1 is supplied to a first mixer 2. The first mixer 2 mixes the RF signal with a frequency signal for selecting a receiving frequency output from a first phase-locked loop (PLL) circuit 6 to convert the reception signal of a predetermined frequency to a first intermediate frequency signal. For example, a reception signal of 900 MHz band is converted into a first intermediate frequency signal of 50 MHz band.
The first intermediate frequency signal is supplied to a second mixer 3 which mixes the first intermediate frequency signal with a frequency signal for frequency conversion output from a second PLL circuit 7 to convert the first intermediate frequency signal into a second intermediate frequency signal different from the first intermediate frequency signal. For example, the first intermediate frequency signal of 50 MHz band is converted into a second intermediate frequency signal of 50 kHz band.
The second intermediate frequency signal is supplied to a demodulator 4, in which it is demodulated according to a predetermined demodulation system. A demodulated baseband signal is developed at a baseband signal output terminal 5.
The first and second PLL circuits 6 and 7 for obtaining the first and second intermediate frequency signals generate the first and second intermediate frequency signals of predetermined frequencies based on the reference frequency signal of a voltage-controlled type temperature compensated crystal oscillator (TCXO) 8.
The accuracy of each of the output frequencies of the first and second PLL circuits 6, 7 depends upon the accuracy of the reference frequency signal output from the voltage-controlled type TCXO 8.
Accordingly, in order to determine the reception frequency with a high accuracy, it is important that the output frequency of the voltage-controlled type TCXO 8 serving as a reference oscillator is constant. However, the output frequency of the voltage-controlled type TCXO 8 is slightly fluctuated depending on some factors such as a temperature.
For this reason, in the receiving circuit, the second intermediate frequency signal output from the second mixer 3 is supplied to a comparator 11. Then, the comparator 11 compares the second intermediate frequency signal with a reference signal (this reference signal has the same frequency as that of the second intermediate frequency signal) supplied thereto from a reference signal input terminal 12 to thereby detect a frequency difference.
A compared result based on the frequency difference detected by the comparator 11 is converted into a voltage signal by a charge pump circuit 13 and then averaged by a low-pass filter (LPF) 14. The voltage signal averaged by the LPF 14 is amplified by an amplifier 15 and supplied to the voltage-controlled type TCXO 8 as a control signal.
An error of an oscillation frequency of an oscillator can be corrected by controlling the oscillation frequency of the voltage-controlled type TCXO 8 as described above.
However, the above-mentioned frequency error correcting circuit is complicated in circuit arrangement.
In the circuit shown in FIG. 1, if a reference signal for comparison is prepared separately in order to enable the comparator 11 to detect the frequency of the intermediate frequency signal, there is then the risk that such reference signal will leak into the intermediate frequency circuit and impose a bad influence upon the intermediate frequency circuit.
Therefore, the frequency error correcting circuit should be designed with great care so as not to affect the intermediate frequency circuit.
Further, a comparator of an edge-detection type may be used as a phase comparator for composing the comparator 11.
When a phase difference signal output from the edge-detection type comparator is smoothed by the LPF 14, the characteristics of the charge pump circuit 13 should become severe upon mass-production.
Although the amplifier 15 is indispensable for increasing a control loop gain of the reference oscillator, it is unavoidable that such amplifier 15 increases a circuit scale.
Taking the aforesaid problems into consideration, it is difficult for the conventional frequency error correcting circuit to correct a frequency powerfully from a circuit scale standpoint. Accordingly, if the receiving circuit is not provided with the frequency error correcting circuit, then a highly-accurate reference oscillator whose oscillation frequency does not fluctuate with an outside influence of change in temperature or the like should be used as the reference oscillator 8.
While the problems the receiving circuit shown in FIG. 1 encountered are described so far, frequency error correcting circuits of other arrangements cannot avoid the aforementioned problems such as the complicated circuit arrangement.